The invention relates generally to inspection techniques and more particularly to a method and system for performing subsurface defect inspections using a multifrequency phase analysis eddy current technique.
A number of different non-destructive inspection techniques have been proposed such as ultrasonic testing (UT), eddy current inspection techniques and surface acoustic techniques. Exemplary eddy current inspection techniques include pulsed eddy current techniques and multi-frequency eddy current techniques. Eddy current techniques are performed to detect anomalies in material surfaces as well as to detect anomalies deep within material surfaces. Eddy current inspection techniques are also used for inspecting coated materials.
It is desirable to inspect forged parts using non-destructive techniques such as eddy current techniques. As is known to those skilled in the art, ultrasonic testing (UT) utilizes a pulse-echo method to detect flaws inside forged materials. However, even with the use of special probes and pulser instrumentation, UT techniques generally cannot detect flaws in the region of approximately 1.5 mm or less near the surface of the material of the part to be inspected. This region is commonly referred to as the “UT blind zone.”
In order to overcome the disadvantages of UT inspection and to add flexibility to the manufacturing process, 0.050 inches (or about 1.27 mm) material is added to the final part configuration. However, the addition of this material generally increases the manufacturing cost due to material waste and production time.
On the other hand, eddy current inspection techniques are capable of detecting surface and near surface defects because the electromagnetic energy is concentrated in a shallow zone at the surface. However, these techniques typically exhibit a poor signal to noise ratio (SNR) for small flaws under the surface due to an “electromagnetic skin effect,” which limits the penetration of eddy currents into the material under test. The pulsed eddy current technique involves eddy current responses over a broad frequency spectrum. However, the pulsed eddy current technique typically requires complicated signal processing developments, as well as high-speed data acquisition and processing systems.
Multi-frequency eddy current techniques concentrate electromagnetic energies at a few frequencies and yield good signal to noise ratios using lock-in amplifiers. A subsurface eddy current multifrequency inspection technique that enhances the signal to noise ratio and improves the near surface detection in deeper locations within the part to be inspected, is therefore desired.